Mice infected with the JHM strain of mouse hepatitis virus (JHM) develop acute and chronic diseases of the CNS. In one model, suckling C57BI/6 (B6) mice are infected intranasally with wild type JHM and nursed by dams previously immunized with live JHM. These mice are protected from acute encephalitis but a variable fraction (40-90%) develops hindlimb paralysis at 3-8 weeks p.i. In all cases, mutations could be detected in the immunodominant CDS T cell epitope recognized in this strain (residues 510-518 of the surface (S) glycoprotein [S510]). During the past funding period, we showed CTL escape correlated with the absence of a strong anti-JHM antibody response in the CNS, the site of infection, and that the lack of CTL escape observed at the other CDS T cell epitope (S598, residues 598-605 of the S protein) was probably due to low binding to the T cell receptor on responding CDS T cells. We also engineered a CD4 T cell escape variant by mutating the immunodominant CD4 T cell epitope recognized in these mice (epitope M133). Remarkably, infection with this virus resulted in a change in mortality from 100 to 0%;re-introduction of a strong CD4 T cell epitope into the CD4 T cell escape virus resulted in increased mortality. Both in JHM-infected mice and in humans and nonhuman primates infected with HIV or hepatitis C virus (HCV), only a fraction of all mutations that confer escape as determined in vitro are actually detected in vivo. The first objective of this proposal will be to determine the basis for this preferential selection, using JHM reverse genetics and structural analyses of Db/peptide S510 and Kb/S598 complexes. As described above, CTL correlates with a suboptimal anti-JHM antibody in the CNS. The second objective will be to prove that a suboptimal antibody response is necessary and sufficient for CTL escape to occur. The final goal will be to determine the basis for the disease attenuation that occurs in CD4 T cell escape variants. Relevance to public health: Virus mutated in CDS T cell epitopes are increasingly recognized as critical in disease progression in patients infected with HIV or HCV, but their evolution is not easily studied in humans. Studies in JHM-infected mice should be relevant for understanding CTL escape in these human diseases. The amelioration in encephalitis that is observed after infection with CD4 T cell escape virus may offer insights into human encephalitis and other CNS diseases, such as MS, which have a T cell component.